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5. Analysis II: Downward propagation from the lower stratosphere to the lower troposphere

STEP 1:

Figure 6: The low-pass filtered time series of ('u_pcm/'t)_r, ('u_pcm/'t)_d, F_m and -F_c averaged over layer 1 (as defined in Fig. 3). Ultra-low frequent variations in F_m are followed by similar variations in -F_c after one week, leading to a positive 'u_pcm/'t in the first part of the winter when F_m and -F_c increase and a negative 'u_pcm/'t in the second part of the winter when F_m and -F_c decrease. F_m seems to induce an equatorward flow in the stratosphere after one week, which associated zonal Coriolis force counteracts F_m.

STEP 2:

Figure 7: The low-pass filtered time series of the meridional wind at 60Ú N integrated over different layers (as defined in Fig. 3). The largest fluctuations in the equatorward flow occur in the stratosphere (layer 1). 'p_0,pcm/'t is proportional to (vdp)₁₊₂₊₃. v₁ and v₂ induce a lower tropospheric poleward flow (v₃), which slightly lags the upper equatorward flow, leading to a negative 'p_0,pcm/'t when -v₁₊₂ and v₃ increase and a positive 'p_0,pcm /'t when -v₁₊₂ and v₃ decrease.

STEP 3:

Figure 8: The low-pass filtered time series of u_pcm and u between 50-70° N, both at 1000 hPa, and the difference between p_0,pcm and p₀ averaged between 40-60° N. Variations in the meridional pressure gradient around 60° N coincide with variations in the surface zonal wind around 60° N and thus in u_pcm near the surface.

STEP 4:

Figure 9: The low-pass filtered time series of v and u², both averaged between 50-70° N, at 1000 hPa. Since the Coriolis force F_c and the zonal frictional force F_w are proportional to v and u², respectively, the almost constant ratio between v and u² indicates a balance between F_w and F_c .